1. Field of the Invention
The invention is generally directed to the art of stereoscopic motion picture projection, and more specifically to a polarization switch or modulator of improved performance characteristics using a wire grid polarizer (WGP).
2. Description of the Related Art
Stereoscopic motion pictures are frequently projected using the polarization method for image selection. An important variation of polarization transmission uses the REAL D/StereoGraphics ZScreen®, which has been described previously in issued and co-pending patents, U.S. Pat. No. 4,792,850 and U.S. patent application Ser. No. 11/430,598. The ZScreen modulator is mounted on a projector, and production units are mounted directly to the body of the projector and hung in front of the projection lens.
An important concern in the ZScreen is the sheet polarizer, which has been available as a product for 60 or 70 years. The sheet polarizer is also called an absorption polarizer because it creates polarized light by absorbing at least half of the visible light. The absorbed light energy goes into heating the polarizer, and for the ZScreen embodiment this requires a cooling fan. Even with the fan the polarizer will fade in time and lose its desirable characteristics because of the fugitive nature of the dyes employed. Without the fan, the polarizer will quickly degrade when used with a modern theatrical theater projector.
The size of the modulator, dictated by the need to dissipate the radiant flux from the projector, precludes mounting the device to the lens itself because of its bulk and weight and thus must be mounted to the projector body. Mounting the modulator some distance from the lens allows for more heat dissipation since the area of the projected beam and thus the flux density is larger at some distance. There are several manufacturers of projectors and each uses a different form-factor, meaning that a different mounting scheme must be devised for each projector. But just about all of the lenses have the same outside dimensions so a light enough ZScreen could be mounted directly on the lens.
To achieve a high contrast ratio (also known as dynamic range), the transmission of dye stuff sheet polarizers must be reduced, and such sheet polarizers have relatively low transmission. Getting light on the screen is a key performance parameter for movie projection. Sheet polarizers with the required contrast ratio have a transmission of approximately 40%. Theoretically, a polarizer of this type could achieve a 50% transmission, but in fact, as noted, dye stuff sheet polarizers do not achieve this level of transmission with the desired contrast ratio performance. In addition, sheet polarizers have some degree of turbidity that reduces image ANSI (American National Standards Institute) contrast, a standard measurement using a checkerboard pattern. ANSI contrast is not to be confused with polarizer contrast ratio, but instead refers to an objective method for determining the range of values from dark to light within the image.
Thus the electro-optics of the ZScreen setup currently need a large surface area so that heat can be dissipated. Accordingly, the ZScreen device, for the reasons given here, becomes bulky and heavy; it requires larger mounting hardware; it requires a fan; and it needs to be held farther from the projection lens to spread out the luminous flux over a larger surface area to promote cooler operation.
Based on the foregoing, it would be advantageous to provide a ZScreen design having reduced size and spacing considerations while providing successful cooling of the device in projection environments. It would also be beneficial to overcome the aforementioned drawbacks present in previously known projection systems.